To answer my own question
Motive's application guide now says the 1118 adapter has been updated to fit 1.8" openings. Has anyone tried it?
I went ahead and ordered the 1118 adapter and it worked. When I first test fit it, it wouldn't hold pressure. I could hear a slow, but steady air leak where the adapter screwed onto the master cylinder even with the thicker gasket. So I was planning to just keep pumping it up. But later when I actually did the job, it held pressure. It might be that I didn't have it fully screwed on the first time or maybe a little bit of brake fluid at the mating point sealed it. Whatever the case, 1118 fit and held pressure up to 15 psi for me.
A few observations:
1. Use the thicker gasket. You'll have to press down on the adapter while turning it to secure it. To avoid damaging the master cylinder, you'll want to have one hand under the master cylinder when you're pressing down on the adapter.
2. I had virtually no air bubbles at the two rear calipers and at the front driver's side caliper. The front passenger's side caliper, however, had a steady stream of air bubbles. Both my wife and I have always felt the brakes were a little soft. Maybe that's why. I haven't driven it yet to see.
3. I don't put any brake fluid in the Motive unit. I only use it to create pressure, which means I have watch the master cylinder reservoir carefully to make sure it never runs out of fluid. The first time I was re-filling the master cylinder, I couldn't see the level of the fluid, so I overfilled. Oops. If you use the Motive unit only for pressure, pour the fresh brake fluid in the master cylinder nice and slow, and have rags or paper towels underneath.
4. I read lots of mixed info about whether to disconnect the battery for some reason related to the ABS. I didn't disconnect it. When I previously flushed the brake fluid out of our 2011 Subaru Outback, I didn't disconnect the battery and it all seemed fine. Hopefully, that's true here. If you KNOW that I should have disconnected the battery or done something else, please let me know. In other words, don't bother to chime in unless you know because you're a Mazda tech or you've seen it in the service manual, not because you read about it in some forum or on a random Youtube video.
5. I bled in the following order: RR, LR, RF, LF. I understand the the CX-9 probably has a diagonally split circuit, so RR, LF, LR, RF might be better, but I didn't notice anything problematic with the sequence that I used.
All the usual caveats about your mileage may vary, blah, blah, blah.
Good luck!